fix(moe): improve error message for GroupedMLP FP8 support

[liuyun7345]

This commit improves the error message when users try to use FP8 with the legacy GroupedMLP implementation. The new message clearly explains:

1. The legacy GroupedMLP only supports bf16
2. For FP8 support, users should use TEGroupedMLP
3. How to enable TEGroupedMLP (--no-moe-use-legacy-grouped-gemm)
4. The required TransformerEngine version (>= 1.9)

Fixes #1564

What does this PR do ?

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[yaox12]

/ok to test f74eee2

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